While electrophotographic photoreceptors using inorganic photoconductive materials, such as selenium or .alpha.-silicon, have been used, organic electrophotographic photoreceptors (OPC) comprising a conductive substrate bearing a photosensitive layer comprising organic photoconductive materials and binder resins have been improved in performances, and their uses are increasing rapidly. Such organic electrophotographic photoreceptors include those of the laminate-type and the single-layer-type, the former having a photosensitive layer having at least a charge-generating layer (CGL), which generates charge on exposure, and a charge-transfer layer (CTL), which transfers the charge, the later having a single-layer photosensitive layer containing a charge-generating substance and a charge-transfer substance both dispersed in a binder resin.
Electrophotographic photoreceptors require that their sensitivity, electric properties and optical properties be accommodated to the directed electrophotographic processes. Particularly, photoreceptors for repeated uses should withstand the electrical and mechanical, external force applied directly on the surface layer, namely the layer farthest away from the substrate (typically, a conductive substrate), during corona electrification, toner development, transfer onto paper, cleaning and so on, to maintain a uniform image quality for a long time. Specifically, they should resist friction which wears or scores the surface, and should be hardly subject to surface deterioration due to the ozone generated during corona electrification at elevated temperatures. To meet such requirements, polycarbonate resins made from bisphenol A or bisphenol Z have been widely used as the binder resins in the photosensitive layer of organic electrophotographic photoreceptors because of their good compatibility with charge-transfer substances and high mechanical strength. However, even these polycarbonate resins are inferior to the layers of inorganic photoconductive materials in durability.
To solve these problems, in Japanese Patent Application unexamined publication No. 4-179961 are proposed polycarbonates containing rigid units of copolymerized biphenol structure. The mechanical strength of the polycarbonates, however, is insufficient for the required abrasion resistance due to the poor tangling of molecular chains.
Crosslinking polycarbonates are proposed as binder resin materials to further improve the durability of electrophotographic photoreceptors. For example, Japanese Patent Application Unexamined Publication No. 4-291348 (1992) discloses crosslinked-type electrophotographic photoreceptors which contain as binder resins crosslinked polycarbonates made from polycarbonates having unsaturated groups in the side chains through crosslinking. However, the structure of the polycarbonates to be crosslinked lacks rigidity, so the layer containing the crosslinked products is too fragile to improve abrasion resistance adequately.
Japanese Patent Application Unexamined Publication Nos. 5-65320 (1993) and 6-41258 (1994) disclose the synthesis of graft polycarbonates by grafting vinyl monomers on polycarbonates having unsaturated end groups. There, however, is no suggestion to use the polycarbonates having unsaturated end groups for the production of the photosensitive layer of electrophotographic photoreceptors, nor to crosslink the polycarbonates to produce crosslinked-type electrophotographic photoreceptors excellent in electrophotographic properties and durability.
Further, binder resins for electrophotographic photoreceptors are generally dissolved in solvents to prepare coating fluid for forming the photosensitive layer, and should not cause whitening nor gelation of the coating fluid. If the coating fluid whitens or sets to gel, the photosensitive layer may crystallize after coating and drying. In the crystallized areas, photo-decay does not occur and the charge remains as a residual potential, which appears as a picture defect.